Module 4
Biological processes & ecosystems
Quick-reference revision notes for parents.
4.1 Photosynthesis
Plants make their own food using sunlight.
carbon dioxide + water → glucose + oxygen
(needs light and chlorophyll)
Glucose is then used for energy (respiration) or stored as starch / made into other substances (cellulose, fats).
4.2 Leaves — adaptations for photosynthesis
- Broad and flat — large surface area for light.
- Thin — short distance for gases to diffuse.
- Chlorophyll in chloroplasts absorbs light energy.
- Stomata on the underside let CO₂ in and O₂ out.
- Veins carry water in and glucose out.
4.3 Plant minerals
Plants take minerals from the soil through their roots:
| Mineral | What it's used for | If lacking… |
|---|---|---|
| Nitrates | Making protein → growth | Stunted growth, yellow leaves |
| Magnesium | Making chlorophyll | Yellow leaves (no chlorophyll) |
4.4 Aerobic respiration
Releases energy from glucose using oxygen. Happens in mitochondria in every living cell — both plants and animals.
glucose + oxygen → carbon dioxide + water (+ energy)
4.5 Anaerobic respiration
Releases energy without oxygen — used when the body can't get oxygen fast enough (e.g. sprinting).
In animals: glucose → lactic acid (+ a little energy)
- Releases much less energy than aerobic respiration.
- Lactic acid build-up causes muscle ache; it's broken down later when oxygen is available (the "oxygen debt").
- In yeast: glucose → ethanol + carbon dioxide — used in brewing and bread-making.
4.6 Food chains and webs
- Arrows show energy flow. Always point from the eaten to the eater.
- Producer — green plant (makes food via photosynthesis).
- Primary consumer — herbivore that eats the producer.
- Secondary / tertiary consumer — carnivores higher up.
- Decomposers (fungi, bacteria) break down dead material and recycle nutrients.
4.7 Disruption to food chains and webs
- If one population is removed, organisms that ate it lose food, while organisms it ate may overpopulate.
- Bioaccumulation: toxic substances (e.g. pesticides) build up along the chain — top predators get the highest dose.
- An invasive species can out-compete native ones, throwing the web out of balance.
4.8 Ecosystems
- Habitat: where an organism lives.
- Population: all the individuals of one species in an area.
- Community: all the populations together.
- Ecosystem: a community plus its physical environment (light, water, soil, climate).
- Environmental change (e.g. drought, pollution, deforestation) can reduce a population — sometimes to extinction.
Quick reference
- Photosynthesis: CO₂ + water → glucose + oxygen (light, chlorophyll)
- Aerobic respiration: glucose + oxygen → CO₂ + water (in mitochondria)
- Anaerobic (animals): glucose → lactic acid; (yeast): glucose → ethanol + CO₂
- Food chain arrows = energy flow, eaten → eater
- Nitrates → protein; magnesium → chlorophyll